1. Field of the Invention
This invention relates to novel cross-linked zein compounds and processes for making articles from the zein. This invention specifically relates to articles made from zein that are resistant to solvents that normally dissolve zein and/or exhibit increased tensile strength, and a single step process using melt processing for their production.
2. Background of the Invention
Zein is a naturally occurring protein derived from corn, and is customarily present in the co-products of the bio-ethanol industry. It can be isolated using various techniques and purchased in substantially pure (greater than 85%) form. Compared to most proteins, zein is characterized by a relative deficiency of hydrophilic groups. In zein, the high proportion of non-polar side chains accounts for the solubility of zein in organic solvents (such as 80% Ethanol/water) and its classification as a prolamine. Zein has been shown to be readily soluble in solvents such as dimethyl formamide (A. Biswas et. al., ACS Symposium Series, 900, Polymer Biocatalysis and Biomaterials, 141-148) and acetic acid [D. Sessa et. al., J. Applied Polymer Science, 105 (5) 2877-2883; G. Selling et. al., Cereal Chemistry, 84 (3) 265-270 and G. Selling et. al., Macromolecular Chemistry and Physics, 208 (9) 1002-1010]. In order to reduce the impact solvent has on zein articles, zein has been treated with cross-linking reagents in solution, such as formaldehyde, where the reaction takes place either before or during the coagulation of the article by using an acid/inorganic salt bath. Additional formaldehyde treatments may be employed. C. B. Croston et al., describe such a process in Industrial and Engineering Chemistry, 37 (12) (1945) 1194-1198. Croston et al. call for zein solutions for spinning containing approximately 13 to 16.5% solids, in the pH range of 11.3 to 12.7. E. T. Cline describes another method for producing zein fibers from basic zein solutions in U.S. Pat. No. 2,475,879. Multiple formaldehyde treatments of zein obtained from a basic zein solution are described in producing a zein fiber by C. D. Evans and C. B. Croston in Textile Research Journal, 19, (1949) 202-211. However, in each of these cases, the zein has been dissolved in solution before treatment with a cross-linking reagent such as formaldehyde.
Recently W. Uy, U.S. Pat. No. 5,750,064, has described a non-basic zein solution which can be dry spun and after post-treatment with formaldehyde gives a resilient article. The articles from these processes have reduced solubility in most of the standard zein solvents. The solids disclosed were 40-60% and required multiple steps.
W. Uy has described the use of zein and water at high temperatures in an extruder to produce fibers. After extrusion the fibers are stabilized by passing them through a formaldehyde containing bath to give articles with reduced solubility (U.S. Pat. No. 5,580,499). However, the % solids described in this technique is very low which would limit the productivity of the process. In addition the process described is a multi-step process.
T. McMeekin et. al. describes the production of zein fibers by extruding a mixture of approximately 55-62% zein, with the remainder being water, through an extruder having a die with holes of the desired diameter. The resulting fibers were then post-treated with formaldehyde by passing them through a bath(s) having suitable reagents. In this process, the maximum zein concentration is 62% and the process requires multiple chemical treatments where the first step is mixing with water followed by multiple ‘hardening’ steps through treatment with formaldehyde.
Y. Yang et. al. describes the use of citric acid for cross-linking zein in solution in the Journal of Biomaterials Science 22 (2011) 1393-1408 and Acta Biomaterialia 6 (2010) 4042-4051. The resulting materials have resistance to water, but there is no mention of resistance to known zein solvents. In addition this process is performed in water which would necessitate the handling of solution waste.
G. Selling et. al. describes the use of glyoxal in solution to cross-link zein in the Journal of Applied Polymer Science, 123 (2012) 2651-2661 and Journal of Biobased materials and Bioenergy, 1 (2007) 281-287. This work requires the use of solvent, necessitating handing solvent waste. The same author describes the use of glyoxal in a melt, without solvent, in a batch process [Journal of Applied Polymer Science, 109 (2008) 2375-2383] and using reactive extrusion [Journal of Applied Polymer Science, 113 (2009) 1828-1835 and U.S. Pat. No. 7,771,528]. These techniques provide zein articles with improved resistance to known good solvents of zein, and can be performed in a melt process which avoids solvents and the handling of large amounts of liquid waste. However, glyoxal is a toxic volatile material.
D. Sessa et. al. describes the use of glutaraldehyde in solution [Journal of Applied Polymer Science 105 (2007) 2877-2883 and Macromolecular Chemistry and Physics 209 (2008) 1003-1011]. This work requires the use of solvent necessitating handing solvent waste. The same author describes the use of glutaraldehyde in the melt, without solvent, in a batch process [Journal of Agricultural and Food Chemistry, 56 (2008) 7067-7075]. As with the processes of G. Selling above, these techniques provide zein articles with improved resistance to known good solvents of zein and can be performed in a melt process thereby avoiding solvents and the handling of large amounts of liquid waste. However, glutaraldehyde is also a toxic volatile material.
Despite these and other advances, the need remains for an improved process for making articles from zein with limited solubility, using less toxic and less volatile reagents.